Commercial methods for producing dicarboxylic acids generally involve oxidizing naphthenes, cycloaliphatic ketones or cycloaliphatic alcohols with nitric acid in the presence of metal oxidation catalysts.
In the case of adipic acid, specific feed materials such as cyclohexane, cyclohexanol and/or cyclohexanone in admixture with nitric acid are heated at about 40.degree.-140.degree. C. in the presence of a catalyst. The resultant oxidation reaction product comprises adipic acid together with small amounts of monocarboxylic acids and dicarboxylic acids and other organic components in admixture with nitric acid and catalyst components. A substantial quantity of the adipic acid product is recovered by cooling the solution and filtering off the crystallized adipic acid. Oxidation methods of adipic acid production are described in U.S. Pat. Nos. 2,439,513; 2,557,281; 2,719,566; 2,840,607; 2,971,010; 3,338,959; and references cited therein.
In a process involving nitric acid oxidation of cyclohexanone and/or cyclohexanol, economically significant amounts of succinic acid and glutaric acid are formed as byproducts in admixture with the adipic acid. After the major portion of the adipic acid is separated by crystallization and filtration, the filtrate mother liquor contains some adipic acid, as well as succinic acid, glutaric acid, nitric acid and metal catalyst values.
Usually this filtrate has been treated as a waste stream. Because of environmental and economic considerations, there has been continuing research effort to develop methods for recovering the valuable and reusable organic and inorganic components of the said filtrate waste byproduct stream.
U.S. Pat. No. 3,726,888 describes a process for the separation and recovery of the components contained in the filtrate waste byproduct stream of an adipic acid manufacturing plant. The filtrate stream comprises a mixture of adipic acid, glutaric acid, succinic acid, nitric acid and metal catalyst values. The separation and recovery process involves contacting the filtrate with alkanol, and extracting with a water-immiscible organic solvent to provide an organic phase containing the formed esters, and to provide an aqueous phase containing the nitric acid and metal catalyst values. Each of the phases is fractionated to separate the mixtures into useful components.
U.S. Pat. Nos. 4,058,555; 4,076,948 and 4,082,788 describe processing improvements which are adapted to overcome some of the difficulties characteristic of the byproduct separation and recovery technology disclosed in the above recited U.S. Pat. No. 3,726,888.
One of the several problems associated with the production of esters of C.sub.4 -C.sub.6 carboxylic acid components (i.e., those acid components contained in the filtrate byproduct stream derived from adipic acid manufacture) is the accumulation of a residual bottoms fraction which results from the fractional distillation procedure for the recovery of refined diesters of succinic acid, glutaric acid and adipic acid.
If the residual bottoms fraction is recycled to the esterification step in the ester production and recovery process, the heavy organic components of the said residual bottoms fraction cause fouling of the esterification and extraction equipment employed in the process. For this reason the described residual bottoms fraction normally is disposed of as a waste stream by burning or dumping.
There remains a need for new technology to improve economic and environmental aspects of adipic acid production by increased conversion of the filtrate waste stream into useful products.
Accordingly, it is a main object of this invention to improve the efficiency of an adipic acid manufacturing operation by recovery of byproduct values.
It is another object of this invention to provide a process for improving the material balance associated with the separation and recovery of C.sub.4 -C.sub.6 dicarboxylic acids contained in a filtrate byproduct stream derived from an adipic acid manufacturing operation involving nitric acid oxidation of cyclohexanone and/or cyclohexanol.
It is a further object of this invention to recover C.sub.4 -C.sub.6 byproducts from an adipic acid manufacturing operation involving nitric acid oxidation of cyclohexanone and/or cyclohexanol, wherein the recovered C.sub.4 -C.sub.6 byproducts are in the form of high purity dicarboxylic acid diesters.
Other objects and advantages of the present invention shall become apparent from the accompanying description and illustrated data.